1. Field of the Invention
The present invention relates to an overheating detection circuit which is formed in the same semiconductor substrate as that of a power device in a power controlling integrated circuit (power IC), and which can detect an abnormal temperature of the power device.
2. Description of the Prior Art
A power IC includes a control region having a logic region and a power device which are formed in the same substrate. Because a power IC is used with high voltages and large currents, an abrupt increase in the consumption of electric power caused by an abrupt change in a load connected to the device or a short circuiting of the load can result in large currents in excess of the rated current flowing through the device, thereby creating a danger that the device will become excessively heated and, in an extreme case, that the device will be destroyed. To protect a power device against such thermal destruction, the temperature of the power device is constantly monitored and, when the temperature of the device exceeds a predetermined temperature or an overheating of the device is detected, some protective action is taken, for example, the power IC is turned-off.
In monitoring a power device, it is preferable to fabricate an overheating detection circuit including a temperature sensor into a substrate in which the power device is formed in order to improve temperature sensitivity and to simplify the circuit arrangement. Typically, however, discrete elements have been used for forming a circuit which interrupts operation of a power device upon detection of over-currents or an overheating of the power device. In this connection, a thermal sensor using a bipolar transistor as a thermo-sensitive sensor is described in E. Habekotte, Bull, ASE/UCS 76 (1985) 5, 9mars, pp. 272-276.
FIG. 1 illustrates a circuit arrangement of an overheating detection circuit of the prior art which uses the thermal sensor described in the above-cited reference. As shown in FIG. 1, a bipolar transistor 91, operating as a thermal sensor, is inserted into a feedback loop of an operational amplifier 92. An external constant current source (not shown) supplies a collector current IC to the transistor 91. Operational amplifier 92 produces an output voltage V.sub.1, which is equal in amplitude but of reverse polarity to a base-emitter voltage V.sub.BE of transistor 91. As shown in FIG. 2, the base-emitter voltage V.sub.BE varies linearly with and is inversely proportional to temperature T. By properly amplifying the output voltage V.sub.1 of the operational amplifier 92 by another operational amplifier 94, an output voltage Vout which varies linearly with respect to temperature T can be obtained.
A feature of the above thermal sensor is that the output voltage Vout varies linearly with respect to change in temperature, and that little error exists over a broad range of temperature change. However, when the thermal sensor is assembled with a power IC as an overheating detection circuit, various problems are encountered. Because the circuit of FIG. 1 uses a constant current circuit with less temperature dependency and a comparator for comparison with the output voltage Vout, a relatively large amount of circuitry is required for the thermal sensor. Further, it is necessary to minimize the temperature dependency of the operational amplifiers 92 and 94, as well as a reference voltage source Vref. Otherwise, an error arising from the large temperature dependency of each of these components will adversely influence the detected temperature. To remove the adverse influence, most of the circuitry except the bipolar transistor 91 is fabricated and contained in a separate package which is placed at a location such that it is not influenced by the temperature of the power IC. Accordingly, when fabricating the overheating detection circuit in a power IC package, it is necessary to solve the problems associated with the substrate temperature and the increased size of the circuit.
The conventional thermal sensor provides an output signal which varies linearly over a broad range of temperature changes. To the contrary, the conventional overheating detection circuit is designed such that when the temperature of a power device reaches approximately 150.degree.-180.degree. C., it determines that the device temperature has reached an overheating temperature, and produces an output signal. Therefore, the overheating detection circuit must produce an output signal which must vary greatly in accordance with a relatively small temperature range. Thus, the performance requirements of the conventional thermal sensor and overheating detection circuit are quite different from each other.